If you have decided you want to abandon the conventional raw file format native to your cameras pictures and you want to adopt the Adobe dng format, Lighroom can complete the conversion for you.
You first have to select the images you want to convert. (1)
Click on Library in the menu bar, then click on Convert Photos to DNG... (2)
Click on the check box to only convert raw files. (3) With this option checked, only the raw files will be converted. If you leave it unchecked, it will convert other file formats, such as your jpg pictures, to dng. Non-Raw file formats will not benefit from being converted so I would not include them.
Next, click on the check box to delete the original raw files after the new converted dng is created. (4) There is really no advantage to converting the file if you are going to keep the old native format file around.
The first drop down menu lets you choose if you want the new filename extension (dng) to be uppercase or lowercase. (5) I don't know why they give you this option; lowercase is the standard format for file extensions, so select lowercase (dng).
The second drop down menu lets you choose what version of Camera Raw you want your new dng files to be compatible with. (6) Select the most up to date version to get the best conversion to dng. The only reason to choose an older version would be if you actually use another Adobe product (Photoshop, or example) or a plug-in that is only compatible with an older version of Camera Raw.
You can then select the size of the preview that you can see in the Lightroom viewer. (7) This is a preference that has no long-term affect and can be reversed. I like to choose full size so that I have the most detailed preview to work with.
The last option is to choose if you want to embed the original raw file inside the new dng file. (8) Embedding your old files into the new will create dng's that are much larger than they need to be. For example, a 13 mb Canon CR2 file converted to dng will result in an 11 mb file. If you select to embed the original raw file, the new dng will be around 24 mb (11 mb of the new plus the 13 mb CR2). The big advantage to converting is to save some space so , just as I suggested to delete the original in step (4) above, I would never choose to check this option.
Click 'OK' and Lightroom will begin processing the files in the background to convert them to dng.